The hidden message says:
The carollers are all (with the exception of the last, of course) primarily:
characters from festive films. The raised studs upon (or behind) which they stand indicate the lengths of their names, and the red studs indicate highlighted letters that contribute towards the message. In all, these characters are:
MARV, KEVIN and HARRY from Home Alone;
RALPHIE from A Christmas Story with 'The Lamp': a table lamp in the shape of a woman's leg wearing a fishnet stocking (a MacGuffin from the movie),
JACK SKELLINGTON from The Nightmare Before Christmas;
JOHN MCCLANE from the Die Hard series,
and (one who definitely does exist outside of movies...) SANTA!
Reading off those highlighted red-letter studs gives us the pertinent festive message MERRY CHRISTMAS!